High pressure pulsed gas sources derived by electrothermal techniques are disclosed, for example, in commonly assigned U.S. Pat. Nos. 4,590,842, 4,715,261, 4,974,487 and 5,012,719. Some of these prior art devices avoid the need for energetic chemicals that frequently become unstable and pose a constant safety problem. In these prior art pulsed gas sources, a capillary discharge is formed in a passage between a pair of spaced electrodes at opposite ends of a dielectric tube, preferably formed of polyethylene. In response to a discharge voltage between the electrodes, a high pressure, high temperature plasma fills the passage, causing material to be ablated from the dielectric wall. High temperature, high pressure plasma gas flows longitudinally of the discharge and the passage through an aperture defined by an electrode at one end of the passage. The gas flowing longitudinally from the passage through the aperture produces a high pressure, high velocity gas jet that can accelerate a projectile to a high velocity. In the '487 patent, the high pressure, high temperature plasma interacts with a propellant mass to produce a high temperature propellant. In the '719 patent, hydrogen is produced by interacting the plasma flowing through the orifice with a metal hydride and some other material to produce high pressure hydrogen. The plasma is cooled by interacting with a cooling agent, for example water, while an exothermal chemical reaction is occurring.
In the '487 patent, the pressure acting on the rear of a projectile is maintained substantially constant while the projectile is accelerated through a barrel bore even though the volume of the barrel bore between the high pressure source outlet orifice and the projectile increases. Such a result is attained by increasing the electric power applied to the capillary discharge in a substantially linear manner as a function of time.
In still a further high pressure pulsed gas source disclosed in commonly assigned U.S. Pat. No. 5,072,647, a high pressure plasma discharge is established between a pair of axially displaced electrodes. The pressure of the plasma in the discharge is sufficient to accelerate a projectile in a gun barrel bore. The plasma is established in a walled structure confining the discharge and having openings through which the plasma flows transversely of the discharge. A chamber surrounding the wall includes a slurry of water and metal particles to produce high pressure hydrogen gas that flows longitudinally of the discharge against the rear of a projectile. To maintain the pressure of the hydrogen gas acting against the projectile relatively constant as the projectile is accelerated down the barrel, electric power applied to the discharge increases substantially linearly as a function of time.
Some concepts employed in the '647 patent have been incorporated into the co-pending, commonly assigned application Ser. No. 08/238,433, filed May 5, 1994. In this co-pending application, a structure establishes at least several axial electrical discharges across axial gaps behind an outlet of a high pressure pulsed gas source, particularly adapted for driving a projectile. The discharges cause plasma to flow with components at right angles to the axial discharges. A conventional propellant mass, e.g. gunpowder, or a hydrogen producing mass, as disclosed in the '647 patent, is positioned to be responsive to the plasma flow resulting from the discharges. In response to the plasma resulting from the discharges being incident on the propellant mass, a high pressure gas pulse is produced.
Those working in the art have recognized that it is desirable for plasma accelerating a projectile to have a maximum amount of pressure close to the base, i.e., rear, of the projectile. Hence, after a projectile is initially accelerated, it is desirable for the power close to the projectile, at the front of a plasma source, to be greater than the power at the rear of the plasma source. However, a problem in producing a plasma with such a power or energy distribution is that pressure waves have a tendency to be produced in the plasma source. The pressure waves from a high pressure plasma source, such as derived from a highly energetic electric power supply (having millions of Joules of energy), can be destructive of a projectile launcher including such a high pressure source. It is, therefore, desirable for a high pressure plasma source having at least several axial electrical discharges to initially produce plasma having about the same power over all of the gaps. After the projectile has moved away from its initial position, it is then desirable for the power applied to the plasma close to the projectile to exceed the power of the plasma farther from the projectile.
A problem with the aforementioned types of devices is that the plasma has a tendency to flow through a plasma confining structure to an electrode needed to establish the axial electrical discharges; the electrode must be at a high voltage relative to metal parts close to it. If the plasma has a high temperature at the time it is incident on the electrode, many charge carriers are incident on the electrode, causing a low impedance electric path to subsist between the electrode and the metal parts. This constitutes a parallel current path to the desired discharges, diverting current away from the desired discharges. The original electric discharges thus have a tendency to be quenched. To overcome this problem in the past, it has been the general practice to design the structure so the electrode is a great distance from the discharge structure. Such an arrangement enables the high temperature of the plasma to be largely dissipated to reduce the number of plasma charge carriers incident on the electrode. However, such a lengthy structure is not conducive to optimum design of cartridges including projectiles adapted to be loaded into military hardware.
Many of these problems are considered and solved in the co-pending, commonly assigned application of Goldstein et al. (Lowe, Price, LeBlanc & Becker docket 277-042), entitled "HYBRID ELECTROTHERMAL GUN WITH SOFT MATERIAL FOR INHIBITING UNWANTED PLASMA FLOW AND GAPS FOR ESTABLISHING TRANSVERSE PLASMA DISCHARGE," filed Oct. 26, 1994. In that application, there is disclosed a high pressure pulsed gas source, particularly adapted to accelerate a projectile along a gun barrel. The source comprises a structure for establishing at least several axial electrical discharges in corresponding axial gaps behind an outlet; the projectile is initially located immediately in front of the outlet. The discharges cause plasma to flow with components at right angles to the axial discharges for a substantial time while the pulse is being derived and while the projectile is traversing the barrel. A propellant mass positioned to be responsive to the plasma flow resulting from the discharges is converted into a high pressure component of the gas pulse by the plasma.
In this prior art structure, the propellant mass is described as gunpowder. Hence, the safety advantages of the earliest electrothermal devices are not included in the structure of the co-pending application. In addition, the prior art use of gunpowder is not particularly efficient because a fraction of the gunpowder burns too late to affect pressure on the projectile base. Also, the electrical energy may be supplied too late into the pulse, that is during a later portion of the pressure pulse when the pressure gradually drops toward zero.
It is, accordingly, an object of the present invention to provide a new and improved method of deriving a high pressure gas pulse, particularly adapted to drive a projectile in a gun barrel.
A further object of the invention is to provide a new and improved method of deriving a high pressure gas pulse from a mass of non-gaseous material that is relatively inert, hence safe, at ambient conditions, and which is vaporized and chemically reacted such that a relatively large percentage of the potential energy thereof is converted to kinetic energy.